J. Limnol., 2013; 72(s2): 1-17 GEOLOGICAL HISTORY DOI: 10.4081/jlimnol.2013.s2.e1 The palaeogeography of Sundaland and Wallacea since the Late Jurassic Robert HALL* Southeast Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK *Corresponding author: [email protected] ABSTRACT The continental core of Southeast (SE) Asia, Sundaland, was assembled from Gondwana fragments by the Early Mesozoic. Con- tinental blocks rifted from Australia in the Jurassic [South West (SW) Borneo, East Java-West Sulawesi-Sumba], and the Woyla intra- oceanic arc of Sumatra, were added to Sundaland in the Cretaceous. These fragments probably included emergent areas and could have carried a terrestrial flora and fauna. Sarawak, the offshore Luconia-Dangerous Grounds areas, and Palawan include Asian con- tinental material. These probably represent a wide accretionary zone at the Asia-Pacific boundary, which was an active continental margin until the mid Cretaceous. Subduction ceased around Sundaland in the Late Cretaceous, and from about 80 Ma most of Sun- daland was emergent, physically connected to Asia, but separated by deep oceans from India and Australia. India moved rapidly north during the Late Cretaceous and Early Cenozoic but there is no evidence that it made contact with SE Asia prior to collision with Asia. One or more arc-India collisions during the Eocene may have preceded India-Asiaonly collision. The arcs could have provided dispersal pathways from India into SE Asia before final suturing of the two continents. During the Late Cretaceous and Early Cenozoic there was no significant subduction beneath Sumatra, Java and Borneo. At about 45 Ma Australia began to move north, subduction resumed and there was widespread rifting within Sundaland. During the Paleogene east and north Borneo were largely submerged, the Makassar Straits became a wide marine barrier within Sundaland, and West Sulawesi was separated from Sundaland but included land. By the Early Miocene the proto-South China Sea had been eliminated useby subduction leading to emergence of land in central Borneo, Sabah and Palawan. Australia-SE Asia collision began, eliminating the former deep ocean separating the two continents, and forming the region now known as Wallacea. The microplate or terrane concept of slicing fragments from New Guinea followed by multiple collisions in Wallacea is implausible. Neogene subduction drove extension and fragmentation of Wallacea that caused both subsidence of deep marine basins and elevation of land; topography and bathymetry changed very rapidly, especially during the Pliocene, but the detailed palaeogeography of this region remains uncertain. Key words: SE Asia, geology, subduction, land, sea. Received: February 2013. Accepted: April 2013. commercial INTRODUCTION DISCUSSION This paper aims to give a brief overview of the late Mesozoic to Early Cenozoic Mesozoic and Cenozoic palaeogeography of the South- Non The core of SE Asia (Fig. 1), Sundaland, was initially east (SE) Asian region, primarily south of Indochina and assembled in the Late Palaeozoic and Early Mesozoic by Thailand. It summarises the main geological events which are discussed in much more detail elsewhere (e.g. Hall, addition of continental fragments carried from Gondwana 1996, 2002, 2011, 2012a) and here the focus is primarily (Fig. 2) to SE Asia (Metcalfe, 2011). From the Jurassic on the palaeogeography illustrated by maps that show the onwards much of Indochina southwards through the Thai- main features of the region during the Cenozoic. Discus- Malay Peninsula and parts of the present Sunda shelf as sion of the many problems in making these palaeogeo- far south as Sumatra was an emergent area of land, prob- graphical sketches can be found in Hall (1998, 2001) and ably largely surrounded by subduction zones, implying other accounts of palaeogeography and geology can be that the margins included volcanoes and mountains. The found in Hall (2009, 2012b). I have attempted to keep the limited geological record that is preserved suggests that number of references to the minimum possible most of the interior of this region was the site of deposi- but all these papers include much larger bibliographies tion of terrestrial clastic sediments during the Jurassic and with more complete discussion of the geological Early Cretaceous (Abdullah, 2009; Racey and Goodall, issues, and several are accompanied by tectonic recon- 2009; Clements et al., 2011). struction animations which are available at http://searg. More continental blocks were added in the Early and rhul.ac.uk/current_research/plate_tectonics/ early Late Cretaceous (Fig. 2) to form what is now much 2 R. Hall of Borneo, West Sulawesi and Java. The fragments had been inferred from nearby unmetamorphosed limestones rifted from Australia in the Late Jurassic and their colli- of Permo-Carboniferous age with fossils of Cathaysian sions ended subduction around the SE Asia promontory characteristics (Fontaine, 1990). There are also Triassic at about 90 Ma. The Woyla intra-oceanic arc of Sumatra sedimentary rocks in Sarawak containing a fossil flora was also added to Sundaland at this time. East Java and with Asian affinities (Vozenin-Serra, 1977). These rocks West Sulawesi are now known to be underlain by Precam- had therefore been interpreted as a Palaeozoic metamor- brian Australian continental crust rather than Mesozoic phic basement overlain by sedimentary rocks (Hutchison, accreted crust (Smyth et al., 2007). South West (SW) Bor- 2005) accreted to Sundaland during the Sarawak orogeny neo was thought to include Palaeozoic metamorphic (Hutchison, 1996) and I had suggested these formed part rocks, and has been regarded as part of a Sunda shield, of a Luconia-Dangerous Grounds block (Hall, 2012a). but our recent work shows that these supposed ancient However, having recently seen these rocks in the field and rocks are largely metamorphosed Cretaceous vol- reconsidered the evidence for the Sarawak orogeny (Hall, canogenic rocks, although we have a few hints of a Pre- 2012a; Hall and Sevastjanova, 2012), I consider it more cambrian basement beneath (Davies et al., 2012). These likely that the metamorphic rocks are not a Palaeozoic discoveries raise the possibility that the Australian frag- basement but instead that Sarawak, the offshore Luconia- ments may have brought with them some terrestrial faunal Dangerous Grounds areas, and Palawan include Asian and floral elements from Gondwana in the Cretaceous. continental material as well as ophiolitic/arc rocks which The SW Borneo rocks have often been correlated with probably represent a wide accretionary zone at the Asia- metamorphic rocks of Sarawak which were also thought Pacific boundary, which wasonly an active continental margin to be Palaeozoic. These too are undated; their age has until the mid Cretaceous. use commercial Non Fig. 1. Simplified geography of Wallacea, Sundaland and surrounding regions. The 200 m bathymetric contour has been used to define the edge of the Asian margin, Sunda shelf and the Sahul-Arafura shelf. Black filled triangles are volcanoes. Lines with small triangles represent subduction zones with triangles on the upper plate. D and S mark the Dent and Semporna peninsulas of eastern Sabah, and ST is the Seram trough. Sundaland-Wallacea palaeogeography 3 Clements et al. (2011) suggested that the termination of Lesser Sunda islands than a South American Andean mar- subduction in the early Late Cretaceous contributed to wide- gin, with locally emergent islands separated from an in- spread uplift of Sundaland, marked by a major regional un- terior Sundaland landmass by shallow seas. To the south conformity. Rocks below the unconformity are Cretaceous and east were deep water oceanic areas of the Pacific and or older, and above are Eocene or younger, representing a Indian oceans. time gap of more than 80 million years (Clements et al., 2011). It is reasonable to suppose that most of Sundaland Cenozoic: Paleogene was an emergent region, and mountainous, supported by the Two geological events are likely to have influenced the presence of Laurasian conifer pollen (Muller, 1968) in distribution of land and sea, and terrestrial biogeographic Sarawak sandstones, from the Late Cretaceous to Middle Eocene. The few rocks of this age that are preserved, notably patterns, in the Sundaland region in the Early Cenozoic. in Sarawak and NW Kalimantan, are predominantly terres- These are the collision of India and Asia, and the resump- trial clastic sediments deposited by rivers, except at the ex- tion of subduction around Sundaland at about 45 Ma. treme margins of Sundaland in Sarawak, Sabah, West India carried a faunal and floral cargo so the age of Sulawesi, and probably offshore East Java, where there are connections with SE Asia which would allow dispersals poorly dated deep marine clastic deposits. A prolonged pe- is very important, but a clear-cut answer to many ques- riod of emergence and widespread reworking of older rocks tions is not easy to provide. Ali and Aitchison (2008) have is suggested by the character of the oldest sediments in the suggested that India could have made contact with the many Cenozoic sedimentary basins that formed throughout Sumatran part of Sundaland in the Paleocene, before the Sundaland when sedimentation resumed in the Eocene. main India-Asia collision.only I see no geological evidence to They are typically quartz-rich conglomerates and sand- support this suggestion, but there is evidence for an India- stones, very mature in terms of grain shape and composition, arc collision at this time, and it is plausible that the arc suggesting multiple episodes of recycling. could have provided a dispersal route into western Sun- At the end of the Cretaceous subduction resumed be- dalanduse long before there were continuous land connec- neath West Sulawesi, and continued through the Pale- tions from India to Asia. The timing of India-Asia ocene (Fig. 3), and there was probably a continental collision remains a controversial issue. There are two margin volcanic arc, probably more like the present-day main views.